Conventional communication satellites provide downlink communication signals to multiple terrestrial communication cells that extend over a geographic region. The downlink communication signals are typically carried over a range of frequency channels within a predefined frequency spectrum or band (e.g., the Ku-band). To prevent interference between downlink communication signals, different ranges of frequency channels are typically directed to adjacent terrestrial communication cells.
For example, U.S. Pat. No. 6,275,479 describes adjacent communication cells as each having allocated to it a fixed sub-band, such as one-third of a full Ku-band spectrum. Selected positioning of the communication cells can prevent adjacent communication cells from utilizing the same fixed sub-band of the available frequency spectrum. This reduces or eliminates interference between communication signals of adjacent communication cells.
An aspect of the present invention is an appreciation that terrestrial regions corresponding to adjacent communication cells can have dramatically different communication bandwidth requirements. In the context of a prior bandwidth allocation of the type described in U.S. Pat. No. 6,275,479, some communication cells may have chronically inadequate bandwidth capacity while nearby cells consistently have excess bandwidth capacity. The conventional fixed bandwidth allocations can limit the extent to which bandwidth resources can be allocated among communication cells.
Accordingly, another aspect of the present invention is a communication satellite in a satellite communication system, the satellite having multiple communication signal transmit horns, for example, that transmit communication downlink signals to multiple corresponding terrestrial communication cells. The satellite includes first and second contiguous, high aspect ratio arrangements of the transmit horns. The first and second arrangements of transmit horns have allocated to them respective first and second orthogonal communication band segments. The first communication band segment is shared and fully allocable among the transmit horns of the first arrangement, and the second communication band segment is shared and fully allocable among the transmit horns of the second arrangement.
The transmit horns, either alone or with reflectors (e.g., parabolic reflectors), are just an exemplary implementation of transmit antennas for transmitting the communication downlink signals. Any other transmit antenna structure could be alternatively used, including phased array structures.
The shared and fully allocable bandwidth of the communication band segment for each arrangement of transmit horns avoids the chronic under-capacity and over-capacity that can arise with fixed bandwidth allocations to adjacent cells. In addition, the high aspect ratios of the first and second transmit horn arrangements result in corresponding high aspect ratios for terrestrial communication cell arrangements that can utilize well the allocable bandwidth of each communication band segment.
In one implementation, the multiple corresponding terrestrial communication cells deliver satellite downlink signals to high aspect or shaped arrangements of cells with aspect ratios of at least about 4:1, for example. Longitudinally adjacent high aspect arrangements of cells receive the full band in respective first and second orthogonal communication formats. With respect to Ku-band communications, for example, each high aspect communication cell arrangement would receive the full nominal 500 MHz bandwidth of the Ku-band. In other implementations, each high aspect communication cell arrangement could receive band segments that are less than the full communication bandwidth.
The full band first and second orthogonal communication band segments delivered to longitudinally adjacent high aspect arrangements of cells may also overlap a majority of each longitudinally adjacent high aspect arrangement of cells. Such overlapping may occur without interference due to the orthogonality of the communication bands delivered to longitudinally adjacent high aspect arrangements of cells. In addition, such overlapping allows the bandwidth of the overlapping communication band to be selectively used in the overlapped arrangement of cells. With overlapping, mutually orthogonal (or otherwise distinguished) full band satellite downlink signal capacity availability, each arrangement can selectively receive up to twice the full band communication capacity, thereby providing a two-time full frequency re-use system. Such an implementation may be characterized as dual-polarization bandwidth allocation and re-use optimization by isomorphic two-color pseudo-mapping.
Additional objects and advantages of the present invention will be apparent from the detailed description of the preferred embodiment thereof, which proceeds with reference to the accompanying drawings.